Early designs of power converters were based on dissipative circuit elements such as bipolar transistors that regulated an output characteristic such as output voltage by controlling the voltage drop across an active circuit element. A circuit element sustaining a substantial voltage drop inherently results in a power converter with generally low power conversion efficiency. To produce substantial efficiency improvements, switch-mode technologies were developed in the 1970s (and earlier) that regulated the output characteristic by adjusting a duty cycle of a switch that is controlled to be either fully on or fully off. In parallel with the introduction of switch-mode circuit topologies, control arrangements were developed to provide precise regulation of an output characteristic by feeding back a signal with an error amplifier to control the switch based on a linearized model of the switch-mode power train that “averages” the switching effects. Such control arrangements generally operate with a clocked, periodic triangular waveform and an error amplifier coupled to an output characteristic of the power converter. The switch in the power converter is turned on at the beginning of each clocked waveform cycle, and remains on until the clocked triangular waveform exceeds an output signal from the error amplifier.
Switch-mode power trains combined with a clocked signal to turn a power switch on have become an industry mainstay, providing excellent and cost-effective results for steady-state operation. Linearized models of switch-mode power trains controlled by such arrangements are generally useful at frequencies substantially below the power converter switching frequency, at frequencies typically lower by an order of magnitude, but they have not provided a necessary response at frequencies near the power converter switching frequency. Recent applications of power converters to loads with abruptly changing power requirements, such as microprocessors that require a step change in a bias voltage or a step change in load current, have challenged the ability of such feedback arrangements to provide a suitably rapid power converter response time.
Roughly in parallel with the development of switch-mode power converters has been extensive research in the field of mathematical optimization in areas such as linear programming, dynamic programming, and the general theory of optimal processes. These developments focus on performance metrics for a system that are generally measured over a time interval, and may explicitly include constraints on an operating parameter of the system such as a limit for the magnitude of a control signal. The performance of a system controlled using a mathematical optimization criterion can far exceed performance obtainable with ordinary feedback control, and is not limited by the familiar control considerations such as phase and gain margins at crossover frequencies. A limiting drawback of such mathematical optimization approaches has been the extensive computation necessary to produce a control signal, especially for a system of modest complexity such as a power supply. The application of control approaches using mathematical optimization has generally been limited to complex systems with critical performance metrics that do not have the cost constraints of high volume, low cost products for a commercial market. Attempts to improve a response of a power converter to abrupt changes in load conditions have generally resulted in a complex and bandlimited control arrangement. A need thus exists for a controller for a power converter that can provide a substantially optimal control signal for an optimization criterion such as a minimum-time objective with sufficient simplicity that it can be implemented with a practical, low-cost circuit arrangement. The controller must be quickly responsive to substantial changes in an output characteristic of the power converter, and, in addition, provide precise steady-state control. A further need is for a controller that can estimate the output state of a power converter with sufficiently fast response time and accuracy so that its rapid response is not compromised. The controller must quickly respond with accuracy to substantial changes in an output characteristic of the power converter without significant lag in an estimate of the output characteristic.